Prevalence of alcohol and other drug (AOD) abuse and criminal activity is highest during emerging adulthood compared to any other developmental period, and causes extraordinary costs to society. They also frequently occur together; Emerging Adults (EAs; ages 17-21) with AOD abuse have greater incarceration rates than EAs without AOD abuse, and AOD-abusing offenders have significantly more recidivism, severe offending, and incarceration than other offenders. Such serious behavior interferes with successful transition into adulthood - school completion, employment, housing. Thus, there is a strong public health need for effective treatment to reduce AOD abuse and justice involvement in EAs. Surprisingly, there are no interventions with established efficacy to reduce criminal activity among EAs, with or without AOD abuse. Among younger adolescents, the comprehensive causes of antisocial behavior are addressed by effective interventions (e.g., Multisystemic Therapy [MST]; Multidimensional Treatment Foster Care [MTFC]), and the present investigative team has developed and evaluated a well-defined age-tailored intervention for EAs with criminal behavior. The developed intervention is an adaptation of MST and integrates a skills coaching component from MTFC, both well-established effective juvenile justice interventions. Initial MST-EA research focused on justice-involved young adults who had mental health problems, a high-risk subpopulation of offenders, but AOD abuse quickly became a primary problem the MST-EA team treated. As a single-source intervention, MST-EA targets the EA correlates of criminal activity and AOD abuse, including gainful EA activities (positive relationships, school, work, and housing) and reduced AOD abuse-in part by targeting the proximal mechanism of poor self- regulation. In a successfully completed community-based open trial, the safety, feasibility, and preliminary efficacy of the intervention were established. The proposed study will evaluate the effectiveness of MST-EA for reducing justice involvement and AOD abuse. EAs (n = 240) with AOD abuse and justice involvement (recent arrests or release from justice facilities) will be randomized to receive MST-EA or Treatment as Usual (TAU). At months 0, 2, 4, 6, 8, 12, and 16, assessments will be completed, with triangulation of all outcome data using system records and collateral reports. Aims will be to evaluate the effect over time of MST-EA for reducing AOD abuse and criminal activity, as well as to evaluate the effect of MST-EA on the key proximal target of treatment (self-regulation) and intermediate outcomes of treatment (gainful activities). A final aim of the study will be to investigate if the direct effect of treatment on criminal activity is mediated by its effect on self- regulation, AOD abuse, and gainful activities. In this specific age group, there is a complete absence of AOD abuse and recidivism reduction treatments with demonstrated effectiveness. The ultimate effect of the proposed research would be decreased AOD abuse and justice involvement in a high-risk population, as well as improved outcomes that have significant societal impact (e.g., reduced homelessness and unemployment).